Client "Address Book" File Download Process http://technet.microsoft.com/en-us/library/bb894482.aspx . If a client is accessing the URL for the first time, upon successful connection, the client attempts to download the current full data file. On subsequent days, the client attempts to download a delta file based on the last full synchronization date.
Typically, this delta file is based on the previous day. If the client is offline for a day or more, it determines which delta files it must download to get up to date. If the client is offline for more than 30 days, it attempts to download the full data file.
Storing this information in a local database reduces the time taken to synchronize information on the client computer with the latest information stored in Active Directory, thereby significantly improving the GAL search process. The location of this database is : Documents and Settings\\Local Settings\Application Data\Microsoft\Communicator.In the event of a download failure because of network connectivity or other issues, the client retries in time intervals that double on each failure (1 minute, 2 minutes, 4 minutes, and so on, up to a maximum of 64 minutes).
Any data that was downloaded before the failure is discarded, and the retry begins again at the beginning. If the failure persists for more than 24 hours, a warning is displayed, and an application event is added to the Event Log.
When the client logs in, it determines if it has been more than 24 hours since the last download. If so, then the current download occurs immediately. Otherwise, download is scheduled at 00:00 UTC (Universal Coordinated Time, also known as GMT).